The present invention relates generally to damping devices. More specifically, this invention relates to a fluid filled elastomeric bushing-type damping device which is used for connecting a vibrating element or assembly, that produces varying types of vibrations, to a rigid support member.
The invention will be described particularly in connection with a fluid filled elastomeric damping device of the fluid bushing type, which isolates an internal combustion engine, such as may be used in a vehicle, from the support frame of the engine. However, it should be appreciated that the invention has broader applications and may be used for the absorption of shocks, structural leveling, and energy dissipation in a variety of other environments as well.
In the typical vibration isolating engine mount, a body of natural or synthetic rubber is normally employed. While these elastomeric mounts can be designed to operate in a generally satisfactory manner, such materials inherently have a low coefficient of damping which limits their ability to isolate certain objectionable vibratory inputs to the vehicle, such as those particularly disturbing to a modern lightweight unitized vehicle body and frame construction. An increased damping coefficient is possible by the selection of certain rubber polymers and the use of additives. This technique has, however, thus far proven unsatisfactory because of accompanying adverse effects on other properties of the rubber. Also, increasing the damping coefficient produces large damping for all vibratory inputs regardless of frequency or amplitude. This is undesirable in an engine mount, particularly when the engine experiences low amplitude, high frequency vibrations.
It is well known that for best performance in a hydraulic elastomeric engine mount, damping should be at a maximum at the natural frequency of the mount system. It is also desirable that the engine mount be able to handle two distinctly different types of vibrations in different ways. More specifically, low frequency vibrations of relatively large amplitude should be damped in such a way that high frequency vibrations of relatively small amplitude remain relatively undamped but are isolated. Unfortunately, a design for successfully damping high amplitude vibrations on the order of 0.3 mm or greater, generally will also damp low amplitude vibrations, on the order of 0.1 mm or less. Various schemes have addressed this problem with some success. Many of the schemes are based on the use of two fluid filled chambers between which is positioned a partition member that is capable of limited free motion.
One of these devices, for example, is an axial or strut-type damping system which provides two fluid filled chambers in which a partition member permits only limited fluid movement between the chambers. Axial damping devices are, however, complex in design, weigh more, and are more expensive to manufacture, as well as needing to be larger in size than engine mounts in the form of bushings. Bushing-type engine mounts are also advantageous over axial mounts for safety reasons. In this connection, bushing-type engine mounts better restrain a vehicle engine against movement during a crash than do axial mounts. Additionally, bushing-type mounts are advantageous over axial mounts since they are better able to damp a rocking motion of the engine. Such a motion is frequently encountered in transaxle mounted engines in front wheel drive vehicles.
Bushing-type damping devices attempting to solve this problem have also been found to be inadequate. More specifically, conventional bushing designs have found difficulty in successfully handling both high frequency, low amplitude vibrations and low frequency, high amplitude vibrations. One known class of bushing-type damping device is capable of successfully handling both of these types of vibrations. However, even this known class is incapable of restraining the amplitude of vibrations of the vehicle engine within desired ranges. Another problem with conventional elastomeric damping devices for internal combustion engines is that due to the general downsizing of cars, less room is available in the engine compartment for the engine. Accordingly, if the vibrations which need to be damped are of too large an amplitude, the engine may contact other elements housed in the engine compartment or the hood with deleterious results.
Accordingly, it has been considered desirable to develop a new and improved vibration damping device in which the damping response and spring rate to structural agitation is varied in two different directions and which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.